The present invention relates to ferromagnetic thin-film structures exhibiting relatively large magnetoresistive characteristics and, more particularly, to such structures used for the storage and retrieval of digital data.
Many kinds of electronic systems make use of magnetic devices including both digital systems, such as memories, and analog systems such as magnetic field sensors. Digital data memories are used extensively in digital systems of many kinds including computers and computer systems components, and digital signal processing systems. Such memories can be advantageously based on the storage of digital symbols as alternative states of magnetization in magnetic materials provided in each memory storage cell, the result being memories which use less electrical power and do not lose information upon removals of such electrical power.
Such memory cells, and magnetic field sensors also, can often be advantageously fabricated using ferromagnetic thin-film materials, and are often based on magnetoresistive sensing of magnetic states, or magnetic conditions, therein. Such devices may be provided on a surface of a monolithic integrated circuit to provide convenient electrical interconnections between the device and the operating circuitry therefor.
Ferromagnetic thin-film memory cells, for instance, can be made very small and packed very closely together to achieve a significant density of information storage, particularly when so provided on the surface of a monolithic integrated circuit. In this situation, the magnetic environment can become quite complex with fields in any one memory cell affecting the film portions in neighboring memory cells. Also, small ferromagnetic film portions in a memory cell can lead to substantial demagnetization fields which can cause instabilities in the magnetization state desired in such a cell.
These magnetic effects between neighbors in an array of closely packed ferromagnetic thin-film memory cells can be ameliorated to a considerable extent by providing a memory cell based on an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic memory thin-film is provided. Such an arrangement provides significant “flux closure,” i.e. a more closely confined magnetic flux path, to thereby confine the magnetic field arising in the cell to affecting primarily just that cell. This result is considerably enhanced by choosing the separating material in the ferromagnetic thin-film memory cells to each be sufficiently thin. Similar “sandwich” structures are also used in magnetic sensors.
In the recent past, reducing the thicknesses of the ferromagnetic thin-films and the intermediate layers in extended “sandwich” structures, and adding possibly alternating ones of such films and layers, i.e. superlattices, have been shown to lead to a “giant magnetoresistive effect” being present in some circumstances. This effect yields a magnetoresistive response which can be in the range of up to an order of magnitude or more greater than that due to the well known anisotropic magnetoresistive response.
In the ordinary anisotropic magnetoresistive response, varying the difference occurring between the direction of the magnetization vector in a ferromagnetic thin-film and the direction of sensing currents passed through that film leads to varying effective electrical resistance in the film in the direction of the current. The maximum electrical resistance occurs when the magnetization vector in the field and the current direction therein are parallel to one another, while the minimum resistance occurs when they are perpendicular to one another. The total electrical resistance in such a magnetoresistive ferromagnetic film can be shown to be given by a constant value, representing the minimum resistance, plus an additional value depending on the angle between the current direction in the film and the magnetization vector therein. This additional resistance has a magnitude characteristic that follows the square of the cosine of that angle.
Operating magnetic fields imposed externally can be used to vary the angle of the magnetization vector in such a film portion with respect to the easy axis of that film. Such an axis comes about in the film because of an anisotropy therein typically resulting from depositing the film during fabrication in the presence of an external magnetic field oriented in the plane of the film along the direction desired for the easy axis in the resulting film. During subsequent operation of the device having this resulting film, such operational magnetic fields imposed externally can be used to vary the angle to such an extent as to cause switching of the film magnetization vector between two stable states which occur for the magnetization being oriented in opposite directions along the film's easy axis. The state of the magnetization vector in such a film can be measured, or sensed, by the change in resistance encountered by current directed through this film portion. This arrangement has provided the basis for a ferromagnetic, magnetoresistive anisotropic thin-film to serve as a memory cell.
In contrast to this arrangement, the resistance in the plane of a ferromagnetic thin-film is isotropic for the giant magnetoresistive effect rather than depending on the direction of the sensing current therethrough as for the anisotropic magnetoresistive effect. The giant magnetoresistive effect involves a change in the electrical resistance of the structure thought to come about from the passage of conduction electrons between the ferromagnetic layers in the “sandwich” structure, or superlattice structure, through the separating nonmagnetic layers with the resulting scattering occurring at the layer interfaces, and in the ferromagnetic layers, being dependent on the electron spins. The magnetization dependant component of the resistance in connection with this effect varies as the sine of the absolute value of half the angle between the magnetization vectors in the ferromagnetic thin-films provided on either side of an intermediate nonmagnetic layer. The electrical resistance in the giant magnetoresistance effect through the “sandwich” or superlattice structure is lower if the magnetizations in the separated ferromagnetic thin-films are parallel and oriented in the same direction than it is if these magnetizations are antiparallel, i.e. oriented in opposing or partially opposing directions. Further, the anisotropic magnetoresistive effect in very thin films is considerably reduced from the bulk values therefor in thicker films due to surface scattering, whereas a significant giant magnetoresistive effect is obtained only in very thin films. Nevertheless, the anisotropic magnetoresistive effect remains present in the films used in giant magnetoresistive effect structures.
A memory cell based on the “giant magnetoresistive effect” can be provided by having one of the ferromagnetic layers in the “sandwich” construction being prevented from switching the magnetization direction therein from pointing along the easy axis therein in one to the opposite direction in the presence of suitable externally applied magnetic fields while permitting the remaining ferromagnetic layer to be free to do so in the same externally applied fields. In one such arrangement, a “spin-valve” structure is formed by providing an antiferromagnetic layer on the ferromagnetic layer that is to be prevented from switching in the externally applied fields to “pin” its magnetization direction in a selected direction. In an alternative arrangement often termed a “pseudo-spin valve” structure, the ferromagnetic layer that is to be prevented from switching in the externally applied fields is made sufficiently thicker than the free ferromagnetic layer so that it does not switch in those external fields provided to switch the free layer.
Thus, a digital data memory cell based on the use of structures exhibiting the giant magnetoresistive effect is attractive as compared to structures based on use of an anisotropic magnetoresistive effect because of the larger signals obtainable in information retrieval operations with respect to such cells. Such larger magnitude signals are easier to detect without error in the presence of noise thereby leading to less critical requirements on the retrieval operation circuitry.
An alternative digital data bit storage and retrieval memory cell suited for fabrication with submicron dimensions can be fabricated that provides rapid retrievals of bit data stored therein and low power dissipation memory through use of a cell structure that has a spin dependent tunneling junction (SDTJ), or magnetoresistive tunnel junction (MTJ), device therein based on a pair of ferromagnetic thin-film layers having an electrical insulator layer therebetween of sufficient thinness to allow tunneling currents therethrough. This memory cell can be fabricated using ferromagnetic thin-film materials of similar or different kinds in each of the magnetic memory films present in such a “sandwich” structure on either side of an intermediate nonmagnetic layer where such ferromagnetic films may be composite films, but this intermediate nonmagnetic layer conducts electrical current therethrough based primarily on the quantum electrodynamic effect “tunneling” current mentioned above.
This “tunneling” current has a magnitude dependence on the angle between the magnetization vectors in each of the ferromagnetic layers on either side of the intermediate layer due to the transmission barrier provided by this intermediate layer depending on the degree of matching of the spin polarizations of the electrons tunneling therethrough with the spin polarizations of the conduction electrons in the ferromagnetic layers, the latter being set by the layer magnetization directions to provide a “magnetic valve effect”. Such an effect results in an effective resistance or conductance characterizing this intermediate layer with respect to the “tunneling” current therethrough. In addition, an antiferromagnetic layer against one of the ferromagnetic layers is used in such a cell to provide different magnetization switching thresholds between that ferromagnetic layer and the other by fixing, or “pinning”, the magnetization direction for the adjacent ferromagnetic layer while leaving the other free to respond to externally applied fields. Such devices may be provided on a surface of a monolithic integrated circuit to thereby allow providing convenient electrical connections between each such memory cell device and the operating circuitry therefor.
A “sandwich” structure for such a memory cell, based on having an intermediate thin layer of a nonmagnetic, dielectric separating material with two major surfaces on each of which a anisotropic ferromagnetic thin-film is positioned, exhibits the “magnetic valve effect” if the materials for the ferromagnetic thin-films and the intermediate layers are properly selected and have sufficiently small thicknesses. The resulting “magnetic valve effect” can yield a response which can be several times in magnitude greater than that due to the “giant magnetoresistive effect” in a similar sized cell structure.
An example of a two state magnetoresistive device structure that is generally common to both of these kinds of memory cells is the “pinned sandwich” structure shown in the layer diagram of FIGS. 1A and 1B where the section line of FIG. 1B defines the view shown in FIG. 1A. This layer diagram gives an indication of the structural layers, but is not a true cross section view in that many dimensions there are exaggerated or reduced relative to one another for purposes of clarity.
A substrate, 2, supports an interconnection structure, 3, as the bottom contact electrode to a magnetic material (ferromagnetic material) free layer, 4, (meaning its magnetization is relatively free to be rotated to an alternative orientation) that is separated by a nonmagnetic material spacer layer, 5, from a magnetic material (ferromagnetic material) relatively fixed layer, 6, (meaning its magnetization is much less free to be rotated to an alternative orientation, i.e. “pinned”). This “pinning” of layer 6 is provided by a further magnetic material layer, 7, the “pinning” layer, that is of an antiferromagnetic material which is magnetically coupled to pinned layer 6 and thereby serves to make this two layer pinned structure relatively resistant to rotation of its initial joint magnetization direction in the presence of moderate external applied magnetic fields. An aluminum cap layer, 8, serves as the device top contact electrode providing a conductive path to a further interconnection, 9.
If spacer layer 5 is an electrical conductor, such as Cu, then the structure will exhibit the giant magnetoresistive (GMR) effect and be termed a “spin valve”. If spacer layer 5 is an electrical insulator, such as Al2O3, that is sufficiently thin, then the device will exhibit the spin dependent tunneling effect and be termed a “magnetic tunnel junction”. In either situation, the electrical resistance of the device is typically higher when the magnetizations of the free and fixed layers on either side of the spacer layer are oriented antiparallel to one another, and is lower when these magnetizations are oriented parallel to one another. The electrical resistance versus external applied magnetic field response characteristic for a spin valve that is measured for sense current being established across the magnetic material layers with the conductive layer therebetween is greater in terms of fractional change than that characteristic measured for the sense current established parallel to these layers because the entire collection of spins in the sense current electrons is forced to interact with both magnetic material layers for the sense current being established across these layers but only a fraction of these electrons interact with both layers for sense currents established parallel thereto.
Plots of the high externally applied magnetic field range and the low externally applied magnetic field range response characteristics of a typical spin valve are shown in the graphs of FIGS. 2A and 2B, respectively. The device resistance versus externally applied magnetic field response characteristics of a magnetic tunnel junction are qualitatively similar. However, the magnitudes of the resistance values and the resistance change values may be quite different. FIG. 2B shows that at moderately high positive externally applied magnetic fields the device resistance is largest, corresponding to the antiparallel alignment of the magnetizations of free and fixed layers 4 and 6; and the device resistance is smallest for moderately high negative externally applied magnetic fields, corresponding to the parallel alignment of the magnetizations of free and fixed layers 4 and 6.
FIG. 3 shows a graph in which the resistance of the device of FIG. 1, either with a conductive or an insulative spacer layer 5, as an approximate fraction of its maximum resistance versus the angle between the magnetizations of free and fixed ferromagnetic layers 4 and 6 on either side of this spacer layer. This relationship is obtained by applying an external magnetic field along the direction indicated by the angle that is larger than the magnetic saturation field of free layer 4 but less than the magnetic saturation field of fixed layer 6.
As stated above, operating magnetic fields imposed externally can be used to vary the angle of the magnetization vector with respect to the easy axis in the ferromagnetic films of these various kinds of memory cell devices, particularly the free layers. Such operational magnetic fields imposed externally can be used to vary the angle to such an extent as to cause switching of the layer magnetization vector between two stable states which occur for the magnetization being oriented in opposite directions along the easy axis of the layer, the state of the cell determining the value of the binary bit being stored therein. One of the difficulties in such memories is the need to provide memory cells therein that have extremely uniform switching thresholds and adequate resistance to unavoidable interjected magnetic field disturbances in the typical memory cell state selection scheme used. This externally applied operating fields scheme is based on selective externally imposed magnetic fields provided by selectively directing electrical currents over or through sequences of such cells thereby giving rise to such magnetic fields so that selection of a cell occurs through coincident presences of such fields at that cell.
In such a coincident current selection arrangement, only that cell in the vicinity of the crossing location, or intersection, of these two paths (one over a sequence of cells and the other through another sequence of cells) experience sufficient magnetic field intensities due to the summing of the fields due to these two currents to cause such a magnetic state change therein. Cells in the array that are located far away from both of these two current paths are not significantly affected by the magnetic fields generated by such currents in the paths because such fields diminish in intensity with distance from the source thereof. Cells, however, located in relatively close proximity to one, but not two, of these two paths do experience more significant magnetic fields thereabout, and those immediately in or adjacent to one such path experience sufficient field intensities to be considered as being “half-selected” by the presence of current in that path intended to participate in fully selecting a different cell along that path at the intersection with the other path on which a selection current is present. Half-selection means that a bit is affected by magnetic fields from the current through one path but not another. Such a coincident interjected magnetic fields memory cell state selection scheme is very desirable in that an individual switch, such as that provided by a transistor, is not needed for every memory cell, but the limitations this selection mode imposes on the uniformity of switching thresholds for each memory cell in a memory make the production of high yields difficult.
As such magnetic thin-film memory cells are made smaller to thereby increase the cell density over the surface of the substrate on which they are disposed, the resulting cells become more subject to magnetic state, or data, upsets due to thermal fluctuations occurring in the materials therein. The depth of the energy well in the magnetic material of such cells can be approximated as Hweff*Ms*Volume, where Hweff is half the effective restoration magnetic field attempting to maintain the current magnetic state following perturbations thereto and so effectively providing the energy well depth, Ms is the saturation magnetization of the magnetic material in the cell, and Volume is the volume of the magnetic material in the cell. In conventional cells, Hweff is provided by shape anisotropy or anisotropy due to the material properties of the cell magnetic material, or both. Typically, the value of Hweff in these cells is less than 100 Oe.
Plotting the magnetostatic energy of a data storage cell magnetic material layer versus the angle between the magnetization and the easy axis of that layer, an energy minimum is seen in the result at the angular value of zero or, with this angle designated as θ, at θ=0 as shown in the graph of FIG. 4. This minimum, having on either side thereof in this plot an energy maximum, that is energy maxima at θ=+90° and θ=−90°, is the “energy well”. The depth of the energy well when no external magnetic fields are applied is simply the difference between the energy minimum and maxima. The value of this energy well can be calculated fromE=½ sin2 θ|{right arrow over (M)}|HkVwhere {right arrow over (M)} is the magnetization, Hk is the anisotropy field, V is the volume, and θ is the angle of {right arrow over (M)} from the easy axis. The magnetization orientation will tend to orient to minimize the magnetostatic energy; i.e,. θ will tend toward zero degrees.
The graph of FIG. 5 shows a Stoner-Wohlfarth switching threshold plot, a portion of an asteroid, and reasonable values of the word and sense fields to provide adequate margins for a memory employing coincident current selection. The solid curve in the figure represents the total field required to cause a bit magnetization to switch from one to the other of two stable states. The total field is the vector sum of the word magnetic field {right arrow over (H)}w due to current provided in an adjacent word line, and the sense magnetic field {right arrow over (H)}s due to current provided through the cell which currents are typically applied along current paths following the two orthogonal axes in the plane of the cell array. The Gaussian curve portion shown in the middle of the plot is representative of the distribution of cell applied magnetic field switching threshold values in an array of real memory cells. The memory array design, then, must account for the varying cell switching thresholds encountered in view of this distribution. As illustrated in the figure, design values for the word and sense fields are about ½ the value of Hk. The remaining energy well depth of those cells half-selected is about ¼ their non-selected depth. This can be shown through calculating the well depth with half selection magnetic fields both present and absent.
The energy expression above, when modified to include the effects of {right arrow over (H)}w and {right arrow over (H)}s, becomesE=½ sin2 θ|{right arrow over (M)}|HkV−|{right arrow over (M)}||{right arrow over (H)}s|V sin θ+|{right arrow over (M)}||{right arrow over (H)}w|V cos θ.Here we assume that {right arrow over (H)}w is parallel to the effective easy axis while {right arrow over (H)}s is perpendicular to this axis. The easy axis is parallel to Hk.
If a half-select word field is applied (i.e. |{right arrow over (H)}w|=½ Hk and |{right arrow over (H)}s|=0), the energy expression becomes:E=½ sin2 θ|{right arrow over (M)}|HkV+|{right arrow over (M)}||{right arrow over (H)}w|V cos θ,where the second term is the energy due to the applied word field. If a half-select sense field is applied (i.e. |{right arrow over (H)}s|=½ Hk and |{right arrow over (H)}w|=0), the energy expression becomes:E=½ sin2 θ|{right arrow over (M)}|HkV−|{right arrow over (M)}||{right arrow over (H)}s|V sin θ,where the second term is the energy due to the applied sense field.
These two equations are plotted in the graphs of FIGS. 6A and 6B. In both cases, the well depth has been reduced by a factor of four, from ½ MHkV to ⅛ MHkV. A physical memory may be designed with slightly different parameters. However, the important factor is the smallest energy well depth for a half-selected cell. The design objective is to ensure that the memory cells are magnetically stable during the data storing, or magnetic state switching, procedure that is repeatedly undertaken with respect to other cells. However, the trade-off between thermal stability and magnetic stability is a serious problem when the total magnetic volume of bits is less than about 105 nm3.
Consider, for instance, a 256 megabit data storage capacity memory cell array provided as part of a monolithic integrated circuit chip organized so as to have 16 byte data blocks (8×17 binary bits) and having implemented therein the well known Hamming single bit error correction code which adds 8 additional bits. A reasonable sub-array to operate would be organized so that the word lines would each be immediately adjacent to 1088 cells to provide magnetic fields thereabout and the sense lines would each connect in series 128 cells for storing and retrieving data binary bits. The data is to be accessed in sequence in two byte groups.
A worst case can be taken to occur if one block had the data therein retrieved continuously for a year. In this case only one block would experience error correction while the remaining 2 million blocks would not. A total of 78 blocks (64−1+16−1) would be continuously half selected. The unselected blocks with 4 times the well depth can be ignored because their thermally induced failure rate would be negligible.
First, consider the situation without any error correction. For randomly occurring failures in a non-redundant system, reliability theory shows that the total failure rate of the 78 half selected blocks is the sum of the individual failure rates. Also each byte that is present in a block is accessed only ⅛ of the time. Thus to achieve a desired 10−5 yearly failure rate for the memory, the failure rate for each cell per ⅛ year is given by10−5/(78 blocks*·128 cells/block)=1.0016×1031 9 failure/(⅛ year).Noting that there are 3.942*106 seconds in ⅛ year, that the relaxation time for a magnetic element of the type considered is about 0.5×10−9 seconds with the energy state being randomized with respect to each relaxation time (tr) so that there are 1/tr chances to fail per second, and that if a cell is thermally excited above the barrier, there is a 50% chance it will end in the wrong state, one can compute the required energy barrier height (Eb) as1.0016×10−9=0.5*e−(Eb/kT)*3.942*106*1/(0.5*10−9)orEb=56.6 kT.
If there is single bit error correction, then any single bit error in any half-selected block can be corrected and a failure occurs at the end of the year only if two or more errors occur in a block. For small failure probabilities, the failure rate for two or more elements failing in a block is only very slightly more than two elements failing in a block. For convenience, calculations will be based on two bits failing in a block.
Let “f” be the cell failure probability for an element for a year. Using the fact that the number of ways 2 elements can be selected out of 138 is given by 138×137/2, the required value of f can be calculated as follows:10−5/(78*69*137)=f2; f=0.3683*10−6The required well depth can then be computed directly as follows:0.3683*10−6=0.5*e−(EbkT)*3.942*106/((0.5*10−9)[half select] Eb=50.7 kT [unselect] 4*Eb=203 kTIncidentally, if the memory has data retrieved from all of the cells therein every 10 hours and corrections made thereto, the required well depth would be reduced by only a relatively a small amount to 47.3 kT.
Taking the more conservative value of Eb=228 kT for unselected memory cells, and assuming the temperature of a half selected cell rises to 400 K during the writing procedure of another cell, and that the saturation magnetization of the storage layer is about 10,000 emu/cm3, and the effective anisotropy of the storage layer is 20 Oe, the minimum volume required to maintain thermal stability is given by the difference between the maximum and the minimum energy values, which occur at θ=0 and at θ=π/2, through the following relationship:½ sin2θ{right arrow over (M)}{right arrow over (H)}kV=228 kT;V=2*228(1.381×10−23 Joules/Kelvin)(400 Kelvin)/(10,000 emu/cm3)(20 Oe)=12.6*10−24 m3=12,600 nm3.
Because of the exponential nature of the threshold, or energy barrier, unselected cells do not contribute significantly to the error rate. Changing the number of half selected cells by a factor of 10 only changes the energy barrier height requirement by 2.3 kT. Although the required barrier energy can be reduced by use of error correction and periodic retrievals with corresponding corrections of the data stored in the whole memory, such a memory takes additional chip area and increased processing steps thereby increasing costs and reducing operating rapidity.
As an example, consider first a memory cell from FIG. 1 with a 0.2 μm width having ferromagnetic free layer element 4 therein of that same width as depicted in that figure. This cell is to be configured to meet the well depth requirements at a temperature of 85° C., i.e. have the necessary half select energy well in the magnetic material, approximated as Hweff*Ms*Volume, where again Hweff is half the effective restoration magnetic field attempting to maintain the current magnetic state following perturbations thereto or Hk/2. In meeting this requirement, ellipsoid shaped like free layer 4 has outer dimensions chosen to be 0.2 μm wide, as indicated above, and to have a length of 0.5 μm so as to have a length to width ratio of n=2.5, with the result that it can the requirement by being provided either of a ternary alloy (65% Ni-15% Fe-20% Co, Ms,=1000 emu) with a thickness of 23.5 Å and a total anisotropy field of 130 Oe or alternatively of a permalloy (80% Ni-20% Fe, Ms=800 emu) with a thickness of 31 Å and an anisotropy field of 124 Oe. In this example, free layer 4 being of such a permalloy is considered in the following.
Assuming that the memory cell is a spin dependent tunneling memory cell, and so having nonmagnetic layer 5 being an electrical insulator to thereby form a magnetic tunnel junction between free layer 4 and pinned layer 6, the cell can be fabricated with free layer 4 directly deposited on a polished, 400 Å thick, copper sense line serving as interconnection structure 3 extending along its length. This sense line is in turn separated by 500 Å of silicon dioxide from a word line therebelow extending along its width. (In a GMR device with nonmagnetic layer 5 being an electrical conductor, the copper sense line would be broken to allow at least free layer 4 to be electrically connected in series with this sense line.) In such a spin dependent tunneling cell structure, the sense current required to generate a 62 Oe half select field is 2.37 mA and the temperature rise from this sense current is 5.3° C. assuming a thermal conductivity for silicon dioxide of 0.014 W/(° C. cm) and a copper sheet resistance of 0.75 Ω/□. For a 0.25 μm thick copper word line clad with 250 Å of permalloy to increase the word field, a current of 3.95 mA is required to generate a 62 Oe word field. With 500 Å of silicon dioxide insulation to the silicon substrate, the temperature rise is less than 0.2° C. for the word line. Current density for the sense line is 3×107 A/cm2 and the current density for the word line is 3.2×106/cm2.
Now consider the result of having the dimensions of this cell reduced through dividing them by some scaling factor greater than one to thereby allow increasing the density of cells in the memory chip. Then thermal stability requires that the energy well depth remain constant or that HkMsV=const. Since the same permalloy material will be assumed to be used for the shrunken free layer, Ms will remain the same and so maintaining HkV constant becomes necessary. However, Hk due to shape anisotropy, which is dominant in small dimension layers, can be written as C1 t/w where C1 is a constant, t is the free layer thickness and w is the free layer width, and the volume V=C2 (nw)wt where C2 is a constant accounting for the ellipsoidal shape and n, as above, is the cell length to width ratio thus allowing the volume to be written C2 w2t by absorbing the constant n into the constant C2. Thus, HkV=(C1t/w)C2 w2t=C1C2t2w=C3. Hence, the initial and scaled values of the product t2w must equal one another, or ti2wi=ts2ws, so that a scaling of the width and length by a factor k giving ws=wi/k requires the scaled layer thickness to become ts=√{square root over (k)}ti with the result that the scaled cell anisotropy field Hk becomes C1k√{square root over (k)}ti/wi=C1k3/2ti/wi, or alternatively, the thickness t must be varied in any such scaling as C3/√{square root over (w)} to maintain HkV constant.
The result, for a scaling factor of two for example, is that the thickness of free layer 4 increases roughly by a factor of the square root of two and the anisotropy field thereof increases roughly by a factor of two times the square root of two. As the external magnetic fields to switch the free layer magnetization are based on providing such fields with magnitudes on the order of Hk/2, and as the path integral around the conductor carrying the current to supply such a field is proportional to that current, the necessary current I in milliamps can be found from Hk/2=(C1/2)k3/2ti/wi=0.4πI[(2wi/k)+2√{square root over (k)}ti] where the terms in the brackets represent the path length around the conductor as a result of the scaling. This expression can be written (C1/2)k3/2=0.4πI [(2wi2/kti)+2√{square root over (k)}wi] so that using, from above, ti=C3/√{square root over (w)}i yields (C1/2)k3/2=0.4πI[(2wi5/2kC3)+2√{square root over (k)}wi]. Thus, the current resulting from scaling is I=(C1/2)k3/20.4π[(2wi5/2/kC3)+2√{square root over (k)}wi] showing that the current I increases with the scaling factor as the width becomes correspondingly smaller. Thus, in this example of a scaling factor equaling two, the sense current through the free layer increases roughly by a factor of the square root of two. As a result, the current density in the layer after scaling, I/[(wi/k)(√{square root over (k)}ti)], and the temperature rise in the scaled layer due to I2R heating therein, I2ρnwi/[(wi/k)(√{square root over (k)}ti)] where ρ is the layer resistivity, increase roughly by a factor of four times the square root of two.
Thus, to meet the thermal stability requirement of maintaining the same energy well depth, the thickness of the resulting 0.1 micron wide free layer 4 in the scaled down cell must be increased to 45 Å. The total anisotropy field becomes 340 Oe and the required sense line current becomes 3.25 mA and the required word line current becomes 4.75 mA. Current density in the sense line increases to 1.63×108 A/cm2 and temperature rise in the line increases to 40° C. These results show the very dramatic increase in current density as elements are reduced in width so that electromigration in the current conductors along with heating must be considered.
As indicated above, a ferromagnetic layer and an antiferromagnetic layer can be deposited in succession so they are in contact with one another with the result that relatively large interatomic forces occur aligning electron spins (parallel for ferromagnetism and antiparallel for antiferromagnetism). These coupling forces at the interface between these layers can be such that the magnetization of the ferromagnetic layer is restored to its initial direction prior to being subjected to external magnetic fields even after very large external magnetic fields are subsequently applied thereto. Such external magnetic fields can be 1000 Oe or more, and the magnetization of the ferromagnetic layer will still be restored to its initial direction. Thus, if such an antiferromagnetic layer is provided in contact with a ferromagnetic layer in a memory cell so that relatively large coupling occurs therebetween, the energy well depth for a small memory cell can be greatly increased. Such an arrangement can increase the potential density of memory cells by more than a factor of 10 through permitting the cell dimensions to go from about 0.2 μm minimum dimensions to approximately 0.05 μm dimensions.
A film structure which exhibits even better resistance to the effects of large externally applied magnetic fields is provided by a compound ferromagnetic thin-film layer with an antiferromagnetic layer. This compound ferromagnetic thin-film layer is provided to have a net layer magnetization that, when fixed in orientation in the finally formed structure, will resist rotation of its magnetization so that the magnetization of this compound ferromagnetic thin-film layer will appear fixed in its orientation in the device, i.e. “pinned” in a direction relative to the finally formed structure.
This compound ferromagnetic thin-film layer is formed by depositing a ferromagnetic layer to perhaps a thickness of 40 Å which is deposited in the presence of an easy axis direction determination magnetic field, then a nonmagnetic layer of ruthenium (no orienting magnetic field needed in this instance) to provide a ruthenium antiferromagnetic coupling layer of 9 Å thickness. Thereafter, another ferromagnetic layer is deposited to a thickness of 40 Å again in the presence of an easy axis direction determination magnetic field aligned as was the field for the first ferromagnetic layer. The resulting compound ferromagnetic layer has materials with high spin polarization in its outer layers due to the use of high magnetic induction ferromagnetic material therein, but has little net magnetic moment because of the ruthenium layer provided therebetween which strongly antiferromagnetically couples these outer layers through primarily exchange coupling (some magnetostatic coupling also present) so that the magnetizations of each are pointed in opposite directions. Thus, this layer is relatively insensitive to externally applied fields and contributes little to the spatial fields thereabout. However, the magnetization direction in this composite layer by itself is not very strongly fixed in any direction because of the relatively weak anisotropy exhibited by the ferromagnetic layers.
Thus, a further antiferromagnetic material “pinning” layer exhibiting a substantial magnetic anisotropy must be deposited on the last ferromagnetic layer to strongly set the magnetization direction of the compound layer. Such an antiferromagnetic layer has a strongly fixed magnetization direction which, through exchange coupling to the last ferromagnetic layer on which it is deposited, strongly fixes the direction of magnetization of that layer also, and so that of the first ferromagnetic layer through the ruthenium layer. The result is an antiferromagnetic layer coupled strongly to the compound layer. Hence, an antiferromagnetic pinning layer is deposited on the last ferromagnetic layer to a thickness of 100 Å or more in the presence of a magnetization axis determination magnetic field aligned with the fields used in forming the two ferromagnetic layers.
If this compound ferromagnetic layer with the antiferromagnetic layer thereon is provided across an electrically conductive layer of perhaps 25 Å thickness from a further ferromagnetic layer of again 40 Å thickness, a good “spin-valve” magnetoresistive memory cell is formed in which this last ferromagnetic layer is the “free” layer which can have its magnetization changed to be either parallel or antiparallel to the firmly fixed magnetization direction of the nearest ferromagnetic layer in the compound ferromagnetic layer to select one of the possible the cell magnetization states (the different states resulting in different cell electrical resistances). This can be accomplished through providing a sufficiently large storage electrical current which will flow primarily through the relatively thick conductive layer between the compound ferromagnetic layer and the “free” layer (although some of this current will also pass through these latter two layers also even though being substantially shunted around by the conductive layer). An external magnetic field directed along the storage current path can also be provided through an appropriately positioned current strap to “tip” the magnetization of the “free” layer to reduce the magnitude needed for the storage current to rotate the “free” layer magnetization. A smaller retrieval electrical current can be directed along the cell current path used for the storage current primarily through the conductive layer between the compound ferromagnetic layer and the “free” layer (though again some of this current will also pass through these latter two layers also despite the substantial conductive layer shunting effect).
This common use of the compound ferromagnetic layer with an antiferromagnetic layer thereon is based on its resistance to alteration of its magnetization direction by externally applied magnetic fields. Omitting the antiferromagnetic layer reduces the ability to set the direction of the magnetization in the compound ferromagnetic layer, but whatever magnetization direction results in the compound ferromagnetic layer in the circumstance of no antiferromagnetic layer being present is still, as indicated above, quite insensitive to externally applied magnetic fields if the two ferromagnetic layers therein are well matched in responding to such external fields. This is true since the effect of an external field on one ferromagnetic layer is directly opposed by the effect on the other because of their magnetizations being held strictly antiparallel to one another by the Ru layer therebetween. Thus, use of a compound ferromagnetic layer without an antiferromagnetic layer thereon would also result in the energy well depth for a small memory cell based on this structure being substantially increased due to the demagnetization fields in each ferromagnetic layer being maintained in directions to approximately cancel one another.
The magnetic fields necessary to reach the layer switching thresholds to cause switching of the relatively fixed magnetization orientation layers magnetization directions for memory cells of smaller and smaller lengths and widths to thereby change the data stored therein have, of course, magnitudes beyond those of the fields required to switch the magnetization directions in the free layers of those cells which also increase for smaller cells as shown above. Generating such magnetic fields begins to require currents through such cells and associated word lines of magnitudes that result in current densities sufficient to cause significant electromigration of the conductive materials and operating temperature rises of the cell region which will alter device behavior and structure. Such effects thereby lead to a limit of some minimum size for these cells.
One possibility for avoiding such limits has been found through allowing memory cell device operating temperature increases due to heating because of supplying word line currents adjacent to, and sense currents in, memory cells sufficient to approach or exceed the Curie temperature of the ferromagnetic layers in memory cells without a “pinning” layer or layers therein, or to approach or exceed the blocking temperature of the antiferromagnetic “pinning” layer in cells having such a layer. Such word line and sense line current based temperature increases permit storage of information in those cells to be achieved without reaching current magnitudes otherwise necessary to switch the magnetization directions of the ferromagnetic layers. The direction of magnetization of the relatively fixed magnetization orientation layer such as the thicker ferromagnetic layer in a three layer “sandwich” structure can be selected by having a moderate magnetic field present oriented in the selected direction when the layer cools sufficiently below its Curie temperature for cells without a “pinning” layer present, or by a field sufficient to set the direction of the ferromagnetic layer adjacent an antiferromagnetic “pinning” layer when that “pinning” layer cools sufficiently below its blocking temperature for cells using such a “pinning” layer or, alternatively, a “pinning” layer composite. The blocking temperature of an antiferromagnetic layer is the temperature at or above which that layer loses its ability to “pin” the magnetization direction of an adjacent ferromagnetic layer below its Curie temperature which blocking temperature is usually less than the Néel temperature of that layer. Similarly, the Curie temperature may not need to be fully reached to allow relatively easy reorienting of the magnetization direction therein.
Reducing the magnitudes of currents necessary for causing the harder ferromagnetic layer in memory cells without a “pinning” layer to approach or reach its Curie temperature, or the antiferromagnetic layer in memory cells with a “pinning” layer arrangement to approach or reach its blocking temperature, and insulating such memory cells from their neighboring cells to provide good cell selectivity in storing information requires providing some thermal isolation of each cell from its neighbors and the integrated circuit substrate or any other kind of substrate serving as a heat sink. Such thermal isolation can be provided by use of electrical conductive interconnections that are of a relatively low thermal conductivity, and by supporting the memory cell on an electrical insulator of relatively low thermal conductivity. However, as the need for increased density of magnetoresistance based memory cells supported in and on a monolithic integrated circuit leads to smaller and smaller cell extents, further structural and operational alternatives for such memory cells are desired.